A conventional oil pump comprises an inner rotor formed with “n” external teeth (“n” is a natural number), an outer rotor formed with “n+1” internal teeth which are engageable with the external teeth, and a casing in which a suction port for drawing fluid and a discharge port for discharging fluid are formed. The inner rotor is rotated to rotate the outer rotor by the engagement of the external teeth with the internal teeth, so that fluid is drawn and is discharged by changes in the volumes of plural cells formed between the inner and outer rotors.
Each of the cells is delimited at a front portion and at a rear portion as viewed in the direction of rotation of the inner rotor and outer rotor by contact between the external teeth of the inner rotor and the internal teeth of the outer rotor, and is also delimited at either side portions by the casing, so that an independent fluid conveying chamber is formed. While the external teeth and the internal teeth engage with each other, the cell becomes the smallest in volume. Then, when the cell moves along the inlet port, it increases in volume to draw fluid, and thereby it has the largest volume. Then, when the cell moves along the discharge port, it decreases in volume to discharge fluid.
Since such oil pumps having the above construction are compact and simply constructed, it is widely used as pumps for lubrication oil in automobiles and as oil pumps for automatic transmissions, etc. When an oil pump is mounted on an automobile, means for driving the oil pump includes a crankshaft directly-connected and driven method in which an inner rotor is directly connected to a crankshaft of an engine and the inner rotor is driven by the rotation of the engine.
With regard to the oil pump as described above, in order to reduce noise emitted from an oil pump and to improve mechanical efficiency accompanied therewith, an appropriate size of clearance is set between a tooth tip of the inner rotor and a tooth tip of the outer rotor in a rotational phase advancing by 180° from a rotational phase in which the inner and outer rotors engage with each other in their combined state.
Meanwhile, the conditions that are required to determine the tooth profile of an inner rotor ri and the tooth profile of an outer rotor ro will be described. First, with regard to the inner rotor ri, the rolling distance of a first circumscribed-rolling circle Di′ (the diameter thereof is φDi′) and the rolling distance of a first inscribed-rolling circle di′ (the diameter thereof is φdi′) must be completed in one cycle. That is, since the rolling distance of the first circumscribed-rolling circle Di′ and the rolling distance of the first inscribed-rolling circle di′ must be equal to the length of circumference of a base circle bi′ (the diameter thereof is φbi′) of the inner rotor ri, the following equation is satisfied:φbi′=n·(φDi′+φdi′)
Similarly, with regard to the outer rotor ro, since the rolling distance of a second circumscribed-rolling circle Do′ (the diameter thereof is φDo′) and the rolling distance of a second inscribed-rolling circle do′ (the diameter thereof is φdo′) must be equal to the length of circumference of a base circle bo′ (the diameter thereof is φbo′) of the outer rotor ro,φbo′=(n+1)·(φDo′+φdo′)
Next, since the inner rotor ri engages with the outer rotor ro, the eccentric distance e′ between the inner and outer rotors ri and ro satisfies the following equations:φDi′+φdi′=φDo′+φdo′=2e 
Based on the respective equations, the following equation is obtained:n·φbo′=(n+1)·φbi′The tooth profile of the inner rotor ri and the tooth profile of the outer rotor ro are constructed to satisfy the above conditions.
Here, in order to divide the clearance t into a tip clearance between a tooth space and a tooth tip in a rotational phase in which the inner and outer rotors engage with each other, and a tip clearance between tooth tips in a rotational phase advanced by 180° from the rotational phase in which the inner and outer rotors engage with each other, the circumscribed-rolling circle and the inscribed-rolling circle are respectively constructed to satisfy the following equations:φDo′=φDi′+t/2, andφdo′=φdi′−t/2
That is, the circumscribed-rolling circle of the outer rotor is made larger than that of the inner rotor (φDo′>φDi). As a result, as shown in FIG. 6, a clearance 2/t is formed between a tooth space of the outer rotor ro and a tooth tip of the inner rotor ri in the rotational phase in which the inner and outer rotors engage with each other. On the other hand, the inscribed-rolling circle of the inner rotor is made larger than that of outer rotor (φdi′>φdo′). As a result, as shown in FIG. 7, a clearance t/2 is formed between a tooth tip of the outer rotor ro and a tooth space of the inner rotor ri in a rotational phase in which the inner and outer rotors engage with each other (For example, see Patent Document 1). Moreover, as shown in FIGS. 6 and 7, not only a tip clearance tt is formed between tip portions of the external and internal teeth of the inner and outer rotors, but also a side clearance ts is formed between the tooth surfaces of the external and internal teeth of the inner and outer rotors.
An oil pump rotor assembly constructed to satisfy the above relations is shown FIGS. 5 to 7. In the inner rotor ri, φbi′=52.00 mm; φDi′=2.50 mm; and φdi′=2.70 mm; and n=10, where φbi′ is the diameter of the base circle bi′, φDi′ is the diameter of the first circumscribed-rolling circle Di′, φdi′ is the diameter of the first inscribed-rolling circle di′, and n is the number of the external teeth, and in the outer rotor ro, φ=70 mm; φbo′=57.20 mm; φDo′=2.56 mm; φdo′=2.64 mm; n+1=11; and e′=2.6 mm, where φ is the external diameter of the outer rotor, φbo′ is the diameter of the base circle bo′, φDo′ is the diameter of the second circumscribed-rolling circle Do′, and φdo′ is the diameter of the second inscribed-rolling circle do′, n+1 is the number of the internal teeth, and n+1 is the eccentric distance.
In the oil pump rotor assembly have the above construction, the inner and outer rotors are formed such that the profile of a tooth tip of the inner rotor is smaller than the profile of a tooth space of the outer rotor and the profile of a tooth space of the inner rotor is larger than the profile of a tooth tip of the outer rotor. Thus, the backlash is set to an appropriate size and the tip clearance tt is set to an appropriate size. As a result, a large backlash can be surely obtained while the tip clearance tt is kept small. Thus, in particular, in a state where the pressure of oil supplied to the oil pump rotor assembly and the torque that drives the oil pump rotor assembly are stable, noise caused by collision between the external teeth of the inner rotor and the internal teeth of the outer rotor can be prevented from being generated.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 11-264381